Dear Graham,
1. Pl check IOS must be same in both 3750 switch.
2. Do one thing Stack cable in switch 1 to STACK 1 ( Which is Master) and switch 2 to STACK 2 (Slave) attach stack cable.
3. Configure in global mode in switch 1
Switch1 config#switch 1 priority 15
Hope to help.
Cheers!!!
Vaib...